RFR: Add build support for generating Jacoco coverage reports [v2]

[Jorn Vernee]

> Add a new `coverage` gradle task that can be used to generate a Jacoco test coverage report. Because we're on the bleeding edge, the user running the build needs to build Jacoco from source for this to work.
> 
> As an example: I've cloned Jacoco from here: https://github.com/jacoco/jacoco and built the project using:
> 
>     mvn package -DskipTests -Dbytecode.version=22
>     
> Then, I point gradle at the generated artifacts using:
> 
>     jacoco_cli=<jacoco repo>/org.jacoco.cli/target/org.jacoco.cli-0.8.12-SNAPSHOT-nodeps.jar
>     jacoco_agent=<jacoco repo>/org.jacoco.agent/target/classes/jacocoagent.jar
>     
> I've put these in a local `gradle.properties` file instead of passing them on the command line.
> 
> I then run `./gradlew coverage` which generates an html coverage report under `./build/jacoco-report`.

Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:

  lazily configure coverage task

-------------

Changes:
  - all: https://git.openjdk.org/jextract/pull/153/files
  - new: https://git.openjdk.org/jextract/pull/153/files/0d019131..83f60913

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=153&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=153&range=00-01

  Stats: 14 lines in 1 file changed: 0 ins; 4 del; 10 mod
  Patch: https://git.openjdk.org/jextract/pull/153.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/153/head:pull/153

PR: https://git.openjdk.org/jextract/pull/153